[mseebach]

A full text index that's actually useful will allow you to largely piece back together the original content, modulo stemming and stopwords.

I guess it would be something like encrypting the index, then decrypt it on demand, just like you would decrypt individual messages on demand.

[swehner]

Not if the index values are encrypted (public-key) too.

hashed-word => encrypted-list-of-msg-indices

something like that.